1. Field of the Invention
This invention relates to a process for the esterification of organo chlorosilanes by contacting the chlorosilanes with alcohol. More especially, this invention relates to a step-by-step esterification of chlorosilanes with alcohols wherein the alcohol contacts the chlorosilane in the liquid phase without contact in the gas phase wherein each step is accompanied by extraction of hydrogen chloride which develops, a final esterification step being carried out endothermically employing an organochlorosilane of the formula EQU H.sub.a R.sub.b SiCl.sub.4-a-b
wherein
R represents an optionally halo-substituted or O-bridges containing alkyl radical or a halogen or a nitro or a protected phenolic group, containing aryl radical PA1 a equals 0, 1, or 2, PA1 b equals 1 or 2, and PA1 a+b amounts to a maximum of 3. PA1 R represents an optionally halogen-substituted alkyl radical which can contain an oxygen or sulfur atom in the chain or a halogen or a nitro or protected phenolic group, containing aryl radical, PA1 a equals 0, 1, or 2, PA1 b equals 1 or 2, and PA1 a+b amounts to a maximum of 3.
2. Discussion of the Prior Art
In the esterification of chlorosilanes, a number of difficulties are encountered which influence the purity and yield of the desired esters as well as those products which are produced coincidentally with the preparation of the ester, notably, hydrogen chloride. Heretofore these difficulties have influenced the reaction in a negative way. In addition to the formation of alkyl chlorides and water as essential products from the reaction, hydrogen chloride also forms alkyl ether when the same contacts an alcohol. Together with the alcohols extracted from the gas phase, the organic compounds pollute the recovered hydrogen chloride and render it unsuitable for the preparation of highly pure trichlorosilane, useful in turn in the manufacture of transistors. The reason for this lies in the fact that, since the hydrogen chloride used to form the trichlorosilane is in itself impure, there develops, in addition to the trichlorosilane formed, impurities such as methyldichlorosilane which cannot be properly removed from the trichlorosilane. The presence of impurities such as methyldichlorosilane means that the transistor silicon produced from trichlorosilane is of low grade and unsuitable for use as a transistor.
In addition to the foregoing problems, the water which develops during a side reaction in the esterification of chlorosilanes by alcohols interferes with the esterification reaction itself by way of hydrolysis. This causes the formation of by-products and residues of oligomeric and polymeric organosiloxanes which are of virtually no use owing to their irregular and undefined composition. These by-products represent a considerable loss of material and diminution of yields. In addition, the maneuverability of the esterification reaction to form pure products is encumbered owing to the fact that the polymeric and oligomeric organosiloxanes often exhibit a stubborn retention for silicon chloride values. This interferes with the alcohol dosage and isolation of pure end products.
Another drawback of the prior art procedures becomes evident during esterification of organochlorosilanes with an O-bridges containing alkyl or alkoxy radical. The hydrogen chloride which develops acts ether-separatingly and yields, amongst other things, an undesirable content of organically bound chlorine within the product. The products developed as a result of such process can contain impurities such as compounds possessing a .beta.-chloroethoxy group which may have high toxic properties.
Some of these difficulties can be overcome by esterification of trichlorosilane according to the procedure of German Offenlegungsschrift No. 24 09 731, the disclosure of which is hereby specifically incorporated herein by reference, wherein esterification is carried out at temperatures below 100.degree. C. by feeding alcohol into the trichlorosilane without contacting the trichlorosilane with the alcohol in the gas phase. The hydrogen chloride which develops is removed from the reaction vessel before a final esterification step. The adherence to a reaction temperature below 100.degree. C. is important in accordance with this manipulative technique. For example, when carrying out the complete esterification with methanol at the boiling temperature of the reaction mixture, the reaction mixture which develops contains so many undesirable by-products that effective distillative removal of the trimethoxysilane which forms is impossible.
Transferring the procedure of German Offenlegungsschrift No. 24 09 731 to the esterification of organochlorosilanes presents the difficulty that only a 50 percent yield of the desired ester is realized and such is realized together with impurities of methanol and methyl chloride within the escaping hydrogen chloride.
It is an object of this invention, therefore, to provide a method for the complete esterification of chlorosilanes whereby the above-mentioned side reactions do not occur or occur only to a minor extent in order to achieve high yields of organosilane esters with practically no gaseous by-products in the developing hydrogen chloride.